X markers are just that, markers you can use for referencing that wont be visible ing ame
They're all persistant references by default, so you don't have to name them. Mostly they're used for telling NPCs where to go or stand. Just use the 'travel' package and direct the NPC to the x marker (easiest way is just by selecting "select reference in render window" and clicking the x marker) and the NPC will head there at the time specified. If you tell the NPC to 'wander' to the x marker, they will head there and wander near the x marker within the radius given
X marker headings are the same, but they have an arrow. The NPC will face the direction of this arrow when they arrive at the marker, so for example it's used for city guard patrols to make them always look forward rather than at the wall. When arrifving at a regular x marker an NPC will just face any random direction